Shape neck: (57 sts) Work both sides at the same time. Work 18 sts. Slip the next 21 sts to a holder. With a second ball of yarn, work remaining 18 sts. Dec 1 st at neck edge every row twice, until 16 sts remain.

So my question is, (the part thats in red) does it mean dec 2 times for each side the left side and the right side on a single row OR dec 1 stitch for each side for 2 rows? i think either way i end up with 16 sts for each shoulder so i'm all confused
thanks!!